Solar Water Pumps — Lab Note 1.01

“Lab Notes” are a new series of posts chronicling the daily progress our research projects. Research Project No. 1 is the testing and installation of a solar heating system for domestic water and space heating. These notes may be useful for anyone interested in building such a system at home. Others might prefer the more succinct guide to solar heating, videos, and other formal publications that will result from this research project and be posted to the website as they are available.


waterheaterdiagram-solararray1The basic premise of a solar heating system is that hot water is circulated through a large panel in the sun, it gets hot, and that heat is stored in a tank for later use. Although some styles of solar water heaters do not require circulation, in an environment that experiences below-zero temperatures, a circulating pump is required. This pump must withstand temperatures above the boiling point. It must have a long use-life. Pumps with variable speeds work better as the warmer the day, the faster the heated liquid should be moved.

We have chosen to use a DC motor because it will be powered by a photovoltaic (PV) solar panel, and it is more efficient if we can avoid the conversion from DC to AC. By using a PV panel wired directly to the pump, the system’s speed will be self-regulating. As more solar radiation hits the solar water panel and the PV panel, the hotter the water and the faster the pump will work.

We have been looking at pumps for a few months now. In our virtual travels, we stumbled across Build It Solar, a website with more comprehensive information for DIY solar applications than we’ve seen anywhere else. They have a section comparing various DC solar hot water pumps. This helped us narrow down our search.

We decided to buy two S5 Solar Hot Water Pumps from US Solar Pumps. We needed two because the distance between the hot water reservoir and the top of the panels is about 14 ft (4.3 m), this is known as the “hydraulic head.” By putting the pumps in series, that is one behind the other, it will almost double their head, which is 7 1/2 ft (2.3 m) each. Note that there is an inverse relationship between head and flow rate. At any pump’s maximum head, it will pump no water. On a flat level, it will achieve its maximum flow rate. Therefore, by having the pumps in series, they should get up to 15 ft (4.5 m), which is higher than necessary, giving us a better flow rate than if we had bought another pump that just meets our head needs.

Although the flow rate is relatively slow (2.2 gallons per minute [GPM] [8.5 liters per minute (LPM)]), once it overcomes the top height, the water flowing back into the reservoir tank will create suction and increase the flow rate, similar to a siphon (if we can figure out the piping for this more-complicated set up).

 


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